Throughout this application various publications are referred to in parenthesis. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications are hereby incorporated by reference in their entireties into the subject application to more fully describe the art to which the subject application pertains.
Subjects with exceptional longevity have generally been spared from major age-related diseases, such as cardiovascular disease, diabetes mellitus, Alzheimer's and cancer, which are responsible for most deaths in the elderly (1). Various studies suggest that while the effect of genetics on life expectancy is minimal across ages, this is not the case with centenarians (a rare phenotype achieved by ˜1/10,000 individuals). Siblings of current centenarians have odds ratios of 8-17 of achieving 100 years of age, and parents of centenarians (born at ˜4870) had odds ratio of ˜7 of achieving ages 90-99 than an appropriate control (2-4). Furthermore, the offspring of long-lived parents had significantly lower prevalence (˜50%) of hypertension, diabetes mellitus, myocardial infarctions and strokes/transient ischemic attacks compared with several age-matched control groups (4-5). In support of the inheritance of longevity, the New England Centenarian Study reported a statistically significant linkage between a genetic locus on chromosome 4 and exceptional longevity among siblings of centenarians (3).
Since lipid profile is directly correlated to cardiovascular disease, a favorable lipid profile may play a pivotal role in longevity. The complexity of such an assumption is depicted with the example of high-density lipoprotein (HDL) levels. The Framingham (8-9) and NHANES III (10) studies have shown that cross-sectional, plasma HDL levels were comparable at different age groups both in males and females. However, looking prospectively, plasma HDL levels seemed to decrease by over 5 mg/dl per decade. These seemingly contradictory results may be explained by HDL being a ‘survival’ factor. A decrease in HDL levels to below a certain range may result in the loss of cardiovascular protection (and possibly protection from other age-related diseases), hence in increased mortality. A study in healthy elderly and centenarians revealed a small but statistically significant reduction in the ratios of cholesterol/HDL-cholesterol and LDL-cholesterol/HDL-cholesterol, and a significant increase in HDL-cholesterol and apoA1 (11). Offspring of centenarians have significant higher plasma levels of HDL levels compared to controls (7). Lipoprotein(a) levels have been reported to be elevated in centenarians at the threshold for atherogenic risk (11-13). Favorable biological markers that are unchanged or decreased in centenarians do not rule out their role in longevity, and some of the harmful lipoprotein profile in centenarians are compatible with healthy longevity, suggesting that other characteristics may protect ‘down stream’ of these pathways.
The metabolic syndrome (MS) of aging/syndrome of insulin resistance is most commonly associated with obesity (115), but may be inherited in lean individuals. This syndrome is commonly associated with dyslipidemia, with decreased HDL cholesterol and increased LDL cholesterol levels, and with decreases in HDL and LDL particle sizes (116). However, it is unclear if increased HDL levels have a role in preventing this syndrome. This syndrome is also associated with hypertension, the development of type 2 diabetes mellitus (117), and a markedly increased risk of developing arteriosclerosis, and is therefore linked to decreased life expectancy (118, 119). Insulin resistance has been identified as a risk factor for a variety of cancers (120-122), broadening its link to shorter life span in humans and to most causes of death (123).
HDL (71) and LDL (22) constitute heterogeneous groups of particles which differ in characteristics such as density, size, electrophoretic mobility, and chemical content. Most of the HDL particles have a globular shape, containing unesterified cholesterol distributed between the surface and the core, and proteins are found in outer parts of the lipoproteins, mainly apoA1 but also apo A-II, A-IV, Cs, E, J, and sphingomyelins (79). Out of five subgroups of HDL, levels of HDL2b have been reported to be increased in a group of 16 centenarian women, while levels of HDL3a are reduced in comparison with controls (72); males were not included in this study. Levels of HDL2-C have been reported to be increased in people 65 years and older (74).
Low blood levels of HDL are strongly related with risk of atherosclerotic cardiovascular disease (57). Overexpression of the major HDL protein, apoA, markedly inhibits progression and even induces regression of atherosclerosis in animal models (40). Decreased plasma HDL level is also a risk for stroke and transient ischemic attack (TIA), but clinical data regarding the effect of increasing HDL cholesterol on vascular events are limited, because its rise is minor and secondary to drugs that lower LDL cholesterol (58, 59).
Amongst the many effects of plasma HDL, it recently became apparent that it may protect from decreased cognitive function associated with Alzheimer's (41) and other forms of dementia (42, 43). In a group of elderly (>85 years of age), the associations between low Mini Mental State Exam (MMSE) scores and low HDL was significant. This relationship was maintained even after subjects with cardiovascular disease or stroke were excluded, supporting the association between HDL and cognitive function independent of atherosclerotic disease (44). Because HDL (and not LDL) has effects that were not clearly limited to the vascular bed, it was recently hypothesized that the very old brain of centenarians, who are not characterized by Alzheimer's disease, may be protected by HDL (14). Indeed, each decrease in plasma HDL tertile was associated with a significant decrease in MMSE.
Previous studies have reported that abnormalities in the LDL receptor are associated with a decreased length of life (60, 61). However, LDL cholesterol has not been reported to change significantly with age in prospective or cross sectional studies (10, 57), although increased age is associated with higher plasma LDL cholesterol and apoB levels in postmenopausal women (57). Studies in healthy elderly and centenarians revealed a small but statistically significant and progressive reduction with age, in total cholesterol, triglycerides (TG) and LDL concentrations, as well as a significant increase in apolipoprotein B100 and lipoprotein (a) values (11-13). Male offspring of centenarians had significant lower plasma levels of LDL-cholesterol and higher levels of HDL-cholesterol compared to controls (7).
LDL particles contain unesterified cholesterol distributes between the surface and the core, and proteins are found in outer parts of the lipoproteins (mainly apoB LDL containing particles). The distribution of mass among LDL subclasses in plasma is reflected by the particle diameter and buoyant density of the predominant LDL species. A distinct LDL subclass pattern characterized by a predominance of small, dense LDL particles (previously called LDL3) has been identified (80). The prevalence of this trait increases with aging, and the prevalence of small particle size LDL (previously called subclass pattern B) is 3-4 fold increased in older compared with young men and women (29, 30). Evidence from several studies is consistent with an autosomal dominant or codominant model for inheritance of the pattern B phenotype with varying additive and polygenic effects (66, 81). One study has reported a predominance of large, buoyant LDL particles in 75% of centenarians and a predominance of small dense LDL particles in 25% of centenarians (73).
The association of plasma LDL cholesterol with a significant risk factor for a variety of cardiovascular diseases is well established (57). The oxidation of LDL is commonly considered to be a major event in the initiation and development of atherosclerosis (62). The plasma lipoprotein profile accompanying a predominance of small, dense LDL3 particles is associated with a 2-3 fold increased risk of coronary heart disease (30, 63-64). More recently, nested case-control analyses in prospective studies of population cohorts have demonstrated that reduced LDL particle size is a significant predictor for the development of coronary heart disease (21, 65-67).
One of the pathways that has been implicated in aging is the insulin/insulin-like growth factor (IGF-1) signaling pathway, which is involved in many functions that are necessary for metabolism, growth, and fertility in animal models as varied as flies, nematodes and mammals (147). Disruption of the insulin/IGF-1 receptor in nematodes and flies increases lifespan significantly, and several mammalian dwarf models live significantly longer, including Snell and Ames dwarf mice and heterozygous knockout mice for the IGF-1 receptor (132, 144). Low IGF-1 levels may protect humans from diseases like cancer (136-140), while normal or high IGF-1 levels may protect humans from osteoporosis (135), diabetes (143), and cardiovascular disease (134). Therefore, based on such data, an overall beneficial effect of changes in IGF-1 levels on human longevity remains uncertain.
Another factor that has been associated with disease process is adiponectin, a protein produced exclusively in adipose tissue, which occurs in serum in relatively high concentration. The plasma concentration of adiponectin is decreased in obese and in type 2 diabetic humans and in patients with coronary artery disease, and low adiponectin levels are a predictor of type 2 diabetes (reviewed in 145, 146). Many clinical reports and genetic studies over the past few years demonstrate decreased circulating levels of this hormone in metabolic dysfunction, such as obesity and insulin resistance, in both humans and animal models. Pharmacologic adiponectin treatments in rodents increase insulin sensitivity, mainly by its hepatic action. This protein also suppresses the expression of adhesion molecules in vascular endothelial cells and cytokine production from macrophages, thus inhibiting the inflammatory processes that occur during the early phases of atherosclerosis.
Cholesteryl ester transfer protein (CETP) is a plasma glycoprotein that catalyzes an exchange of cholesteryl esters (CE) and TG between HDL and APOB containing lipoproteins (70). The atherogenic properties of CETP have been demonstrated by blockade of CETP in cholesterol-fed rabbits, an animal with elevated CETP activity and high atherosclerosis susceptibility (82, 83). However, studies in CETP-deficient patients did not clarify whether CETP is atherogenic (84). CETP exerts a strong and direct effect on HDL size. Expression of CETP in normolipidemic rodents has a profound effect on large sized HDL, which was suggested as a reliable index of low plasma CETP activity in vivo (48, 85). In humans, plasma levels of large HDL particles from patients homo- and hetrozygous for CETP deficiency increased two- and six-fold while levels of small HDL remained unchanged (45, 46). The CETP 405 valine allele is associated with increased levels of HDL (55, 86). The presence of the B2 allele at the Taq1B polymorphism in intron 1 of the CETP gene has been associated with increased HDL particle size (106). Complete CETP deficiency causes a small-sized LDL population with low affinity for the LDL receptor (47). However, because an up-regulation of the LDL receptor increases LDL clearance, CETP deficiency is characterized by lowered LDL levels (49). Conflicting observations have been reported between CETP mutations and the incidence of coronary heart disease (CHD). Increased HDL cholesterol levels caused by mutations in CETP were associated with a slight increased risk of CHD in white Danish women (53). Similar observation of increase in CHD was observed in Japanese-American men with hetrozygous CETP D442G missense mutation (87), though their HDL levels were 10% increased. However, recently the Veterans Affairs HDL Cholesterol Intervention Trial reported that CETP TaqI B2B2 genotype is associated with higher HDL cholesterol levels and lower risk of CHD in men (54).
Other genes are also involved in lipoprotein metabolism. Of particular interest is the gene encoding apolipoprotein C-3 (APOC-3). Transgenic APOC-3 mice are hypertriglyceridemic, and ‘knock out’ of this gene results in hypotriglyceridemic mice (124). APOC-3 is an effective inhibitor of very low-density lipoprotein (VLDL) TG hydrolysis, has a regulating role on uptake of cholesteryl esters, and may have a role as an inhibitor for lipoprotein lipase (LPL), although its exact role is not fully understood. Polymorphisms in APOC-3 have been associated with strong effects on triglyceride levels (125-127). The APOC-3 promoter region has conferred protection against or susceptibility to severe hypertriglyceridemic. The cysteine (C) allele of the Cysteine(−641)Alanine (A) polymorphism, the C allele of the C-482Threonine(T) polymorphism, and the T allele of T(−455)C polymorphism are protective against hypertriglyceridemia (128-129). Increased incidence of the C allele in the T-455C polymorphism was noted with advanced age, indicating that this variant promoter is associated with longevity (130). Furthermore, APOC-3 has effects on lipoprotein size through displacement of apolipoprotein E (APO-E) (131).
Despite the advances in knowledge discussed above, there remains a clear need for markers of longevity which may be used to decrease the risk of developing age-related diseases including dementia and metabolic- and cardiovascular-related diseases.